The Prior Art
Aromatic diamino compounds are well known as building blocks for high performance polymers (polyamides, polyimides, etc.), antioxidants, intermediates for dyestuffs and a variety of other applications. However, the production of the diaminodiphenyl compounds leads, in most cases, to considerable amounts of side products as a result of side reactions and degradation under the production conditions.
The diaminodiphenyl compounds to which the present invention is directed are selected from: ##STR4## wherein R.sub.1 and R.sub.2 can be: H, --CH.sub.3, --C.sub.2 H.sub.5, --C.sub.3 H.sub.7, --C.sub.4 H.sub.9 or ##STR5## Y can be: C(CH.sub.3).sub.2, ##STR6## n=0;1.
The purification of diaminodiphenyl ethers is described in detail in JP 01,272,558. In this patent crude 3,4'-diaminodiphenyl ether (3,4'-DADPE) is purified by carrying out two consecutive distillations: a first, rough distillation to remove the heavy and high boiling materials, followed by a second, fine distillation.
It is claimed that this process leads to purification in high yields and in the high purity which is necessary for certain polymers. However, this is achieved by a double distillation and the use of large distillation columns, which lead to undue operational and economic drawbacks.
The patent gives an example of 30 theoretical plates to distill 3,4'-diaminodiphenyl ether in 99.99% purity. Moreover, when operating according to the process disclosed in JP 01,272,558 some additional technical problems arise in the first rough distillation, e.g. the heavy residues are removed from the distillation system with great difficulty, as they are highly viscous and insoluble in organic or aqueous solvents.
In the above patent, the prior art is critically discussed in detail:
Recrystallization of diaminodiphenyl ether is disclosed in U.S. Pat. No. 4,539,428 and JP 61,221,157. However, such an operation is expensive and troublesome, and the yields of the recovered products and solvents are relatively low.
Conversion into dihydrochloride salts followed by neutralization is disclosed in U.S. Pat. No. 1,890,256 and in JP 61,225,155. However, this operation is associated with large amounts of waste (2 equivalents of salts, such as NH.sub.4 Cl, per one mole of product).
Different approaches are revealed in the following two patents:
a) distillation and fast cooling of diaminodiphenyl ether in JP 61,05,056, which is criticized in JP 01,272,558 as being insufficient to achieve the high purity that is required for high performance polymers. The need to quickly cool the distillate according to this process suggests that, indeed, it still contains impurities that darken the product at the elevated temperature; and PA1 b) distillation in the presence of an inert organic solvent in JP 61,221,159, which leads to unnecessary solvent losses and/or to the need for additional facilities to separate and recycle the solvent. PA1 a. The fractional distillation of the diaminodiphenyl-compounds can be carried out in one step, as a result of the substantial reduction in the decomposition of the compounds in the reboiler. PA1 b. The distillation range of temperatures and pressures is significantly higher (based on the concentration of the corresponding diaminodiphenyl-compound in the crude material) and the purity of the distilled product is considerably higher even if the distillation is conducted under less exacting conditions and using less efficient columns (i.e. smaller number of theoretical plates). In fact, in some cases, i.e., when the product is very sensitive, the fractional distillation method of purification is not economically viable unless NaOH or KOH are used. PA1 c. The distilled product is completely stable and does not darken on storage for long periods (1 year). PA1 and PA1 d. The high boiling residue after the distillation is easily washed off by dissolving it in water.